(i) Technical Field
The present invention relates to a toner for electrostatic latent image development used for forming an image by electrophotography such as a copying machine, a production method thereof, and a developer for electrostatic latent image development using the toner for electrostatic latent image development.
(ii) Related Art
In recent years, due to advances in information technology and increased environmental concerns, more space-saving, resource-saving color image printers having reduced size and weight are required for business use. For this reason, there have been advancements in speed and adaptability to plain paper in color image printers such as inkjet, thermal transfer and sublimation printers, which are fundamentally easy to reduce the size and weight thereof.
However, an electrophotographic system is still predominantly used in offices for its inherent properties such as high speed printing and adaptability to plain paper, even with the difficulty in reducing the size and weight thereof compared to the above-described type of printers. In the electrophotographic system, a tandem-type image forming device, equipped with four image forming units of different colors respectively comprising a photosensitive body and a developing equipment which perform latent image formation, development and transfer, is a typical image forming apparatus which prints color images at high speed.
Since this apparatus has the image forming units of respective colors, images of C (cyan), M (magenta), Y (yellow) and K (black) can be formed almost simultaneously, and thereby printing speed can be improved. However, since four image forming units having the same function are used, the apparatus cannot be reduced in size and weight.
Therefore, in order to enable color image formation in a single image forming unit, a toner which develops necessary colors with one kind of toner particle has been proposed. A fundamental principle of the color development of the toner is to react a dye precursor with a developer by means of an external stimulation corresponding to an image information, thereby forming the necessary colors.
The toner utilizes the technique of a recording paper which is pre-coated with an ink layer containing microcapsules which are responsive to an external stimulation such as light or heat.
For example, there have been suggested particles in which plural microcapsules having a capsule wall which changes the substance permeability thereof when subjected to an external stimulation are dispersed and mixed into a toner resin. In the particles, one of two kinds of reactive substances (dye precursor of respective colors) which cause a color developing reaction when mixed with each other is contained in microcapsules, and the other (developer) is contained in a toner resin which is outside the microcapsules.
By using a photoisomerizing substance which increases the substance permeability thereof when irradiated by light having a specified wavelength as a capsule wall, or by using a capsule wall which is destructed when a supersonic wave of a resonance frequency is applied, the toner forms a color due to the reaction of two kinds of reactive substances existing inside and outside the microcapsules, when light or a supersonic wave is applied.
On the other hand, as a process of preparing a toner, a process called a wet method such as a polymerization method, an in-liquid drying method and an emulsion aggregation method has recently been put into practical use from the viewpoint of environmental concerns and further improvement in functions. As these methods of preparing a toner do not require a high temperature and a high degree of shearing force as compared with conventional methods such as a kneading-pulverizing method, materials which have been conventionally difficult to utilize for a toner can also be used.
Although a toner which develops a color when an external stimulation such as light is applied has an advantage of being capable of forming a favorable full color image using an apparatus having a simpler construction and reduced size and weight, as compared with a case where the conventional toner containing a coloring agent such as a pigment is used, specific preparing process thereof is not practically described. That is, actually, a toner of a type that responds to an external stimulation to form a color without using a coloring agent is yet to be realized.
It is reported that, when a capsule wall of microcapsules contains a photoisomerizing substance, progression of color formation can be stopped by applying a light stimulation again to close an opening of a capsule wall, after the image is formed.
However, the substance permeability of a capsule wall comprising a light-responsive photoisomerizing substance is controlled by utilizing a photoisomerization reaction. For this reason, when a toner is stored for a long period of time, there is a possibility that a toner spontaneously forms a color. In addition, since the image is exposed to light stimulation such as an indoor fluorescent lamp or sunshine after the image is formed, it could also be expected that opening of the capsule wall will occur again depending on the intensity or wavelength of light. Therefore, when microcapsules which have a possibility of reversibly responding to a light stimulation, even after the image is formed, are contained in the image, there is a large possibility that a once formed image will change in color and deteriorate the color balance thereof.
Increase in substance permeability of a capsule wall caused by a photoisomerization reaction utilizes transition from the trans state to the cis state. However, such kind of reaction is reversible and a reaction of returning to the trans state from the cis state could occur at the time of image formation. Therefore, sufficient color formation may not be obtained in some cases.
Furthermore, when a capsule wall is destructed by an external stimulation such as a supersonic wave to form a color, color formation proceeds under an environment such as high temperature even after the image formation. Thus, there is also a possibility of deterioration in color balance of an image.